Solar cells are now widely used, and the competition among manufacturers on the performance of solar cells is heating up. Many types of solar cell composition have also been developed, including single crystal silicon, amorphous silicon, thin film silicon and organic compounds. In order to fairly evaluate the photoelectric conversion efficiency of these solar cells, methods for evaluating solar cells are defined by IEC 60904 and JIS (C8905 to C8911). IEC is an acronym for International Electro technical Commission, and JIS is an acronym for Japanese Industrial Standards.
Since a solar cell has a unique spectral response characteristics due to material and structure, the photoelectric conversion characteristic of a solar cell greatly depends on the spectral irradiance of the irradiation light used for performance evaluation. Therefore as mentioned above, the evaluation method is defined by standards. Generally the performance of a solar cell is measured indoors using a solar simulator that has spectral irradiance L (λ) approximated to the spectral irradiance of the reference sunlight (=S (λ)) under standard test conditions agreed on internationally.
However the solar simulator is constructed by combining a xenon lamp and an optical filter, for example, and it is extremely difficult to approximate the illumination light thereof to reference sunlight. FIG. 6 is a graph depicting an example of the spectral irradiance S (λ) of the reference sunlight, according to the above-mentioned IEC 60904. FIG. 7 is a graph depicting an example of the spectral irradiance L (λ) of a solar simulator. The graph in FIG. 8 shows a result of integrating the spectral irradiance S (λ) in FIG. 6 and the spectral irradiance L (λ) in FIG. 7, which have different wavelength regions and illumination levels. In FIG. 8, the graph indicated by a reference symbol α1 is the spectral irradiance S (λ) of the reference sunlight, and the graph indicated by a reference symbol α2 is the spectral irradiance L (λ) of the solar simulator.
Therefore in order to reproduce the reference sunlight, Patent Document 1 discloses a solar simulator which generates light having a similar spectrum of the sunlight, from ultraviolet to infrared, by selectively transmitting/reflecting light, using a mirror having wavelength dependency, out of lights from a plurality of light sources (xenon lamp and halogen lamp) which emit lights in a mutually different wavelength range, and combining the transmitted/reflected lights.
Patent Document 2 discloses a technique to correct changes of the light quantity of a solar simulator, which measures the irradiance of the light source, and matches the response characteristic of the irradiance measurement sensor with the response characteristic of the solar cell itself, so as to cancel the changes of the light quantity of the solar simulator. Non-Patent Document 1 below will be referred to in the later mentioned embodiments.
The above-mentioned techniques are all calibration methods for a single solar simulator. However solar simulators differ depending on the manufacturer, and have machine differences even if the manufacturer is the same, therefore if a solar cell is measured using a different solar simulator, the electric power generation changes even if the solar simulator satisfies the above-mentioned characteristics.
Therefore the measurer sends a solar cell sample and requests the National Institute of Advanced Industrial Science and Technology (a national institute that provides an internationally unified reference sunlight spectrum, or an equivalent institute), for example, to perform measurement. In response to this request, the institute determines the short-circuit current Isc of the sample at natural sunlight AM 1.5 and 100 mW/cm2 using its own solar simulator that can infinitely approximate light to the reference sunlight, and returns the sample with the measured value (=A) to the measurer. Then the measurer uses the returned sample as the reference cell at their company, and adjusts the light quantity of the solar simulator. In other words, the measurer adjusts the light quantity of the solar simulator first using the reference cell so that the short-circuit current Isc becomes A, then measures the characteristic of the actual measurement target solar cell (inspection target product). As mentioned above, it is difficult to accurately reproduce the optical spectrum of the reference sunlight, but with this technique, the solar simulators of each company can match the reference sunlight as accurately as possible.
In order to complete calibration using the reference cell according to this technique however, it is necessary for the measurer to create a sample and send it to a public institute, where the institute measures the sample and sends it back. This takes time and is costly. Furthermore, calibration of the solar simulator is not completed all at once, but must be repeated with creating a new reference cell every time the spectral responsivity of the solar cell to be measured changes, hence this takes enormous time and cost.
The inventor of the present invention therefore proposed a method for evaluating a solar cell which does not require the reference cell, by measuring the spectral responsivity P (λ) of the measurement target solar cell in advance, and converting the short-circuit current, due to the reference sunlight with the spectral irradiance S (λ) byES=EL·{∫S(λ)·P(λ)dλ}/{∫L(λ)·P(λ)dλ}where EL is the short-circuit current due to the irradiation light by the solar simulator with the spectral irradiance L(λ) (PCT/JP 2009/066105 (W0/2010/058649A1); published on May 27, 2010).
The above technique, however, is effective for crystalline solar cells of which spectral responsivity P (λ) is stable, as in the case of single crystalline silicon solar cells, but an error is generated if applied to solar cells having characteristics where the spectral responsivity P (λ) changes depending on the irradiated light quantity, as in the case of thin film solar cells (e.g. amorphous, fine crystal, compound, dye-sensitization, organic).
Patent Document 1: Japanese Patent Application Laid-Open No. H8-235903
Patent Document 2: Japanese Patent Application Laid-Open No. 2004-134748
Non-Patent Document 1: J. Metzdorf, “Calibration of solar cells: the differential spectral responsivity method”, Applied Optics, May 1, 1987, Vol. 26, No. 9, pp. 1701